Heat release rate is the single most important parameter determining the fire hazard of a material. Several different bench scale methods of determining the heat release rate of burning specimens have been developed and are in use. All of these bench scale methods require samples on the order of 100 grams and thermal diffusion in these thick specimens dominates the thermal history of the material. Moreover, results from these bench scale tests depend not only on the sample mass and thickness but also on the spatial orientation of the specimen, boundary conditions, ignition source, and other parameters of the test setup totally unrelated to the properties of the material. Consequently, the flammability parameters determined in these devices are operationally-defined extrinsic quantities and not the intrinsic properties needed by materials scientists to develop fire resistant polymers.
A thermogravimetric analysis (TGA) technique for determining flammability characteristics of milligram samples of polymeric materials has been reported which uses the TGA sample furnace as the combustion reactor. Carbon dioxide (CO.sub.2) and carbon monoxide (CO) generation are measured and used to calculate heat release during aerobic pyrolysis of 50 mg polymer composite samples during slow transient heating in the TGA furnace. Although this thermogravimetric technique probably represents the first published attempt to directly measure heat release rate during combustion of milligram samples, the method suffers from several problems as a flammability test: 1) Aerobic pyrolysis conditions and the high surface to volume ratio of the specimen used in the published TGA method result in rapid and complete volatilization of oxidizable organic material to gaseous products leaving no residual char. Total thermo-oxidative degradation as occurs in the published TGA method does not represent the thermochemical fuel generation process in burning materials. Burning materials are characterized by a pyrolysis zone in which anaerobic reducing conditions exist to produce a residual carbonaceous mass (char) in aromatic or cyclic polymers. As char production is an important and well known mechanism for reducing the flammability of polymers, it is important to reproduce the necessary conditions leading to char formation in order to have a meaningful flammability test. 2) The TGA flammability technique utilizes a heating rate of only 20 K/min and this is well below the 600 K/min pyrolysis zone heating rate estimated for polymer burning. Thermogravimetric studies have shown that the mass loss history or fuel generation rate of char-forming polymers is significantly different at heating rates of 600 K/min than at 20 K/min and cannot be predicted from mass loss kinetics obtained at the lower heating rate. Consequently the rates of polymer fuel generation and heat release measured in the published TGA flammability technique at low heating rates would be significantly different from those which occur in a fire involving the same material. 3) Only time-averaged values for the heat release rate calculated from CO.sub.2 and CO production in the relatively slow non-flaming aerobic TGA thermo-oxidative decomposition experiment are used to calculate heat release rate in the thermogravimetric technique. The relationship between this average heat release rate per unit area of sample pan in the TGA method and bench-, intermediate-, or full-scale heat release rate tests of materials and components is obscure. 4) Since oxidative combustion reactions occur in the TGA furnace at the pyrolysis temperature there is no flame temperature oxidative combustion step in the published TGA method as occurs in the combustion zone of burning polymers where volatile anaerobic decomposition products from the pyrolysis zone mix with oxygen to produce a diffusion flame. 5) Carbon dioxide/carbon monoxide generation calorimetry, although more sensitive than oxygen consumption calorimetry, is inherently less accurate and requires more equipment and detailed knowledge about the chemical composition of the sample material. 6) The heat flux at the sample surface in the published thermogravimetric method is poorly defined compared to bench-scale fire tests where constant, calibrated, heat fluxes are employed.